First, our new covenant means Christians should be looking at the teachings of Christ for their rules of life.
Second, David did not write all the Psalms, but even if he did, 3000 year-old poetry sheould not be used to promote the assasination of the president of the United States.
Third, David was a deeply flawed man who committed many horrific acts, the most notorious being his rape of a woman and the subsequent sending her husband off to die in battle when the woman told David his rape had impregnated her. Calling David “a man after God’s own heart” simply repeats the political propaganda of David’s time.
So, go read your Bible (all of it, not just the parts you like) and talk with your clergyman (if you have one) about your confusion, providing his name isn’t Anderson.
If Christ is God, then wouldn't all the teachings of God in the Old Testament be the teachings of Christ?
Did God change?